1. Field of the Invention
The present invention relates to a Pb-free solder which is usable as a substitute for the conventional Pb--Sn eutectic solders used at a soldering temperature of 220-230.degree. C. for connecting LSI, parts, etc. on organic substrates and which has a sufficient reliability in mechanical strength even at a high temperature of 150.degree. C., and to an electronic product made using the solder.
2. Description of Related Art
Most of the generally used substrates for printed circuit boards are made of glass fabric based-epoxy (hereinafter referred to as merely "glass-epoxy"). The maximum heat resisting temperature of glass-epoxy substrates is 220-230.degree. C. when a reflowing furnace is used. The solders used for connecting electronic parts on the substrates are Pb-63% Sn eutectic solders (melting point: 183.degree. C.) or solders having near the eutectic composition. Since melting point of these solders is about 183.degree. C., they can perform sufficient connection at a temperature lower than the heat resisting temperature of general-purpose glass-epoxy substrates (230.degree. C.). Furthermore, as for the reliability of mechanical strength at high temperatures, a mechanical strength up to 150.degree. C. at the maximum can be guaranteed.
Recently, it has been reported that in U.S.A., printed circuit boards used for electronic parts are exposed to the weather, and lead (Pb) contained in the solders readily reacts with acids (accelerated by acid rain) and dissolves in underground water, which exerts a bad influence upon human bodies when it is used as drinking water. Under the circumstances, as soldering alloys free from Pb to be substituted for the Pb alloys, alloys of Sn, Zn, Bi, etc. have been noticed as the suitable alloys which exert less influence upon environment, have less toxicity on human bodies, have less problems of draining of resources, have less problems in cost, and have been actually used as materials. As binary solders, Sn-3.5% Ag (melting point: 221.degree. C.) and Sn-5% Sb (melting point: 240.degree. C.) have already been actually used as Pb-free solders. However, these cannot be used for soldering on glass-epoxy substrates because of their too high melting point. Sn-9% Zn (eutectic of 199.degree. C. in melting point) has a lower melting point, but the surface is very susceptible to oxidation and is considerably inferior to Sn--Ag and Sn--Sb alloys in wettability with Cu or Ni. Further, the melting point is not so low as electronic parts being able to be reflowed onto generally used glass-epoxy substrates at 220-230.degree. C. It is empirically known that the soldering temperature is higher 30-50.degree. C. than the melting point of solders. For example, in the case of Pb-63% Sn eutectic solder (melting point: 183.degree. C.), the standard maximum reflow temperature in the furnace is 220.degree. C. The difference between the melting point and the soldering temperature is 37.degree. C. In the case of wave soldering which is carried out in a short time, the standard maximum temperature is 235.degree. C. The temperature difference is 52.degree. C. When wettability is poor, this temperature difference must be larger. When an Sn-9% Zn solder is used, it is known that it hardly wets at a reflow temperature of 230.degree. C. even if generally employed rosin fluxes (chlorine content: 0.2%) are used.
Furthermore, there are Sn--Bi based solders (typical composition: Sn-58% Bi; melting point: 138.degree. C.) and Sn--In based solders (typical composition: Sn-52% In; melting point: 117.degree. C.), but high-temperature strength at 150.degree. C. of these solders cannot be guaranteed. Therefore, these compositions cannot be said to be substitute for Pb-63% Sn eutectic solders, and development of new soldering materials in new combinations satisfying the demands has been desired.
Ternary solders comprising Sn--Zn--Bi as main components are hopeful from the point of melting point. The Sn--Zn--Bi solders are disclosed in JP 57-11793(A) and 59-18906(A).
JP 57-11793(A) proposes low-melting point solders for Al which comprises 5-10% of Zn and 8-13% of Bi, the balance being Sn, and which is excellent in corrosion resistance. These are solders for Al aiming at improvement of strength at low temperatures and not for connection to printed circuit boards having Cu conductors. Furthermore, since they contain more than 5% of Zn and are vigorously oxidized to form a strong oxide film, the oxide film cannot be reduced with low-activity fluxes (rosin type) which are generally employed for the connection of electronic parts. Accordingly, those fluxes which contain organic acids or inorganic or strong active agents must be used. Use of such high performance fluxes for connecting electronic parts causes corrosion due to the remaining flux. For example, for the connection of connector pins, the flux entering inside the connector cannot be removed by cleansing and remains as a residue, which causes corrosion of conductor portions. Therefore, as a package, a flux containing up to 0.2% of chlorine has generally been used. If fluxes of the higher performance are used, there may be the problems such as corrosion with the fluxes remaining even after cleansing, occurrence of migration and deterioration of electric insulation characteristics, and such fluxes are not used at present.
JP 59-189096(A) proposes solder alloys which comprise 5-15% of Zn and 3-20% of Bi, the balance being Sn, aiming at improvement of strength in connecting wires. The alloys used in Examples have high melting points and cannot be said to have solder compositions with which reflowing can be carried out at lower than 230.degree. C. which the glass-epoxy substrates can stand. Furthermore, since they contain more than 5% of Zn and undergo considerable oxidation to form a strong oxide film, the oxide film cannot be reduced with the low performance fluxes (rosin type) which are generally employed for the connection of electronic parts. Accordingly, high performance fluxes must be used. However, use of such high performance fluxes for connecting electronic parts causes problems such as corrosion with the remaining fluxes and deterioration of electric insulation characteristics, and such fluxes cannot be used.